The global market for efficient power generation equipment has been expanding at a rapid rate since the mid-1980's. This trend is projected to continue in the future. The gas turbine combined cycle power plant, usually comprising a gas turbine based topping cycle and a Rankine based bottoming cycle, continues to be the preferred choice in power generation. This may be due to the relatively low plant investment cost as well as the continuously improving operating efficiency of the gas turbine based combined cycle.
During operation of a gas turbine, there may occur a phenomenon known as compressor stall, wherein the pressure ratio of the compressor initially exceeds some critical value at a given speed, resulting in a subsequent reduction of compressor pressure ratio and airflow delivered to the combustor. Compressor stall may result from a variety of conditions, such as rapid acceleration of the engine, or undue distortion of the inlet profile of air pressure or temperature during normal operation of the engine. Compressor damage due to the ingestion of foreign objects or a malfunction of a portion of the engine control system may also result in a compressor stall and subsequent compressor degradation. If compressor stall remains undetected and permitted to continue, the combustor temperatures and the vibratory stresses induced in the compressor may become sufficiently high to cause damage to the gas turbine.
Elevated firing temperatures enable increases in combined cycle efficiency and specific power. Furthermore, for a given firing temperature, an optimal cycle pressure ratio may be identified which maximizes combined-cycle efficiency. This optimal cycle pressure ratio has been theoretically shown to increase with increasing firing temperature. Axial flow compressors, which are at the heart of industrial gas turbines, are thus subjected to demands for ever-increasing pressure ratios, with the simultaneous goals of minimal parts count, operational simplicity, and low overall cost. Furthermore, an axial flow compressor is expected to operate at a heightened level of cycle pressure ratio at a compression efficiency that augments the overall cycle efficiency. An axial flow compressor is also expected to perform in an aerodynamically and aero-mechanically stable manner over a wide range in mass flow rate associated with the varying power output characteristics of the combined cycle operation.
The operating compressor pressure ratio of an installed gas turbine engine is typically set at a pre-specified margin away from the surge/stall boundary, generally referred to as surge margin or stall margin, to avoid unstable compressor operation. Uprates on installed base and new products that leverage proven technologies by adhering to existing compressor footprints often require a reduction in the operating surge/stall margin to allow higher pressure ratios. At the heart of these uprates and new products is not only the ability to assess surge/stall margin requirements and corresponding risks of surge, but also the availability of tools to continuously predict and monitor the health of the compressors in field operations.
One approach monitors the health of a compressor by measuring the air flow and pressure rise through the compressor. A range of values for the pressure rise is selected a-priori, beyond which the compressor operation is deemed unhealthy and the machine is shut down. Such pressure variations may be attributed to a number of causes such as, for example, unstable combustion, or rotating stall and surge events on the compressor itself. To detect these events, the magnitude and rate of change of pressure rise through the compressor are monitored. When such an event occurs, the magnitude of the pressure rise may drop sharply, and an algorithm monitoring the magnitude and its rate of change may acknowledge the event. This approach, however, does not offer prediction capabilities of rotating stall or surge, and fails to offer information to a real-time control system with sufficient lead time to proactively deal with such events.